The present invention is directed generally to an improved fluid bed furnace and more particularly to a new and improved fuel supply system for use in a fluid bed furnace.
Furnaces for heat-treating metal components or the like are well known. Such furnaces have included a heated bath taking the form of melted salt or molten lead. The salt or molten lead baths are heated to an elevated temperature and the metal components to be treated are immersed therein, whereupon, the heat of the baths is transferred to the metal components for treatment.
Such furnaces, while being generally successful in heat-treating metal components or the like, have exhibited certain difficulties. For example, heated molten salt is hazardous when employed because it is potentially explosive should water come into contact with the heated salt. The salt may be heated to a temperature of, for example, 1400.degree.-2000.degree. F., which, upon contacting water, will explode. Hence, a great deal of care is necessary when employing such a furnace to make sure that water does not come into contact with the salt baths.
In the case of molten lead baths, molten lead is extremely acidic in nature, and hence, it is difficult to use in terms of finding a suitable container in which to confine the molten lead. Furthermore, molten lead and salt are wetting materials which as a result wet the metallic components being heat treated. As a result, some means must be provided for removing the molten lead or salt bath materials from the component subsequent to being treated.
In view of the foregoing, a furnace for heat-treating metal components or the like has been proposed which does not include salt or molten baths. In contrast, this type of furnace utilizes fluidizable material which may take the form of corrundum or silica particles. One material which becomes fluidized upon being subjected to a sufficient flow of heated gas and which has become popular for such use is sand.
Such furnaces, have become known as fluid bed furnaces, and one such furnace is fully described in U.S. Pat. No. 3,884,617, which patent is incorporated by reference herein. The particulate fluidizable material is confined within a container known as a fluid bed. Below the fluid bed is a burner which burns a fuel mixture usually comprising a mixture of natural gas and air in substantially stoichiometric proportions. Upon burning the fuel mixture, the burner provides a heated gas mixture which is at an elevated temperature and which flows upwardly and heats and fluidizes the fluid bed. The heat from the gas mixture is transferred to the fluidizable material within the fluid bed so that metal components placed within the fluid bed in turn receive heat from the fluid bed for heat treating.
Furnaces of this type have become increasingly popular because they provide relative high heat-transfer efficiency and do not exhibit the shortcomings previously alluded with respect to the salt and molten lead bath type furnaces. More particularly, fluidizable materials such as sand are inert and therefore will not corrode or eat away at the walls of the fluid bed containing the fluidizable material. Also, upon being heated, the fluidizable material such as sand is not explosive and therefore its contact with water need not be a source of concern. Should water come into contact with the heated fluid bed, the water will merely be converted to steam without a dangerous condition being present. Lastly, the sand fluidizable material is non-wetting. As a result, metal components treated therein need not be processed subsequent to the heat treating for the purpose of removing the fluidizable material.
As usually encountered in the use of such a furnace, the heat-treated components will remove a small portion of the fluidizable material. After prolonged use, such as a matter of days, a sufficient number of components will be treated such that the level of the fluid bed will be decreased by a significant amount. Should the burner continue to heat the fluidizable material at the same BTU rate, excessive agitation of the fluidizable material particles will result. Such excessive agitation can cause the fluidizable material to actually escape from the top of the fluid bed.
It is therefore a general object of the present invention to provide a new and improved fluid bed furnace which precludes excessive agitation of the fluidizable material even when the level of the fluidizable material within the fluid bed decreases.
It is a more specific object of the present invention to provide a new and improved fuel supply system for a fluid bed furnace which senses the level of the fluidizable material within the fluid bed and in response to such sensing, varies the rate in which the fuel mixture is supplied to the furnace burner in direct relation to the level of the fluidizable material within the fluid bed to prevent excessive agitation of the fluidizable material.